Cavity former

ABSTRACT

A cavity former  10  for use in forming a cavity in a concrete slab is disclosed. The cavity former comprises a support member which supports the weight of a person standing on the cavity former or of wet concrete bearing down on the array until the concrete has cured sufficiently to support itself. The array is reconfigurable between a compact state for stacking with other similarly configured arrays to facilitate transport and an expanded state to define a volume about which the wet concrete is poured. The cavity former  10  has a cover  34  which is engageable with the array to distribute the weight onto the array elements and prevent wet concrete flowing into the volume defined by the elements in the expanded state.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Australian patent application serial no. 2003903688, filed Jul. 16, 2003. The disclosure of the above application is incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to concrete slabs and foundations. More specifically, the invention relates to an apparatus for forming a hollow cavity in a slab or foundation.

BACKGROUND OF THE INVENTION

The practice of using cavity formers as space filling members in the preparation of a concrete slab or foundation is an existing technique for reducing the amount of concrete required to form a slab or foundation. While the reduced concrete content in a slab having hollow cavities serves to reduce the cost of the slab, it also advantageously allows the slab to be built on unstable soil as expanding soils will flow into the formed cavities. Such cavities also assist the easy placement of electrical and plumbing conduits throughout the slab.

As the cavity formers must be sufficiently strong to support, firstly, the weight of workers when the formers are in place prior to a pour and, secondly, the load of wet concrete when it is poured, they are prefabricated remote from the building site in standard sizes and then delivered to the building site ready to be set in place prior to pouring of the concrete. Known cavity formers include multiple web reinforced cardboard or fibreboard boxes and so called pods of foamed polymeric material, for instance polystyrene.

Cardboard or fibreboard formers must have sufficient structural integrity to support the wet slab for a prescribed period, e.g. 3 hours, after a concrete pour, before they weaken through moisture absorption. Polystyrene overcomes this limitation, but it has the disadvantage that, when cut into smaller shapes for cavities of smaller sizes, large quantities of fine polystyrene particles are often generated. Such polystyrene particles present an environmental problem because they are easily scattered by a breeze. Furthermore, polystyrene pods do not disintegrate to more completely form the cavity and this presents a disposal and environmental problem when the slab is partially or wholly demolished or reshaped. For these reasons, the use of polystyrene as a cavity former is being restricted in at least the United States.

It is an object of this invention to provide an improved cavity former which is of optimum versatility but retains the strength necessary to support the wet concrete during pouring and the weight of workers beforehand.

SUMMARY OF THE INVENTION

In accordance with the invention, it has been recognised that transport of cavity formers would be more convenient were the formers, collectively, to have a less voluminous state than their volume ready for the concrete to be poured. This less voluminous state is achievable by providing a suitable structural configuration which is stackable and which also enables the cavity former to meet the weight support requirements mentioned above. In a particularly advantageous application of the invention, such a structural configuration permits the formation of the cavity former from a degradable material.

A cavity former, in accordance with a first aspect of the invention, is provided for use in forming a cavity in a concrete slab, said cavity former comprising:

-   -   (a) a support member for supporting a predetermined weight of a         person standing on the member or of wet concrete bearing down on         the member until the wet concrete has cured sufficiently to         support itself; and     -   (b) cover means detachably engageable with the member for         distributing said weight onto the member and preventing flow of         said concrete into the member;     -   wherein said support member is stackable with at least one other         similar support member for transport, the stacked support         members collectively defining a volume less than the summed         volume of the separate members.

In one particular embodiment of the first aspect of the invention, there is provided a cavity former for use in forming a cavity in a concrete slab, said cavity former comprising:

-   -   (a) an array of elements for supporting a predetermined weight         of a person standing on the elements or of wet concrete bearing         down on the elements until the wet concrete has cured         sufficiently to support itself;     -   (b) cover means detachably engageable with the array for         distributing said weight onto the array elements and preventing         the flow of wet concrete between said; and     -   (c) means for locating the cover means in engagement with the         array;     -   wherein said array of elements is reconfigurable between a         relatively more compact state for stacking with other similarly         configured arrays to facilitate transport of the array and an         expanded state for engagement with said cover means, the         expanded state defining a volume about which the wet concrete         may be poured thereby creating a cavity in the resultant         concrete slab.

Preferably, means is provided for retaining the cover means in engagement with the array, e.g. a snug fit between the cover means and the array, and/or means biasing the array into firm engagement with the cover means.

Said elements of the array are advantageously panels that are elongate horizontally in situ, arranged with their planes parallel or orthogonal in the expanded state.

The elements, in the expanded state, may define a plurality of cells which are divisible from the cavity former to thereby alter the size and shape of the cavity former without affecting the integrity of the volume defined by the remaining cells.

In one particular embodiment, the elements pivotally intersect with each other such that the array is foldable between the more compact and expanded states. In another embodiment, the elements include elements formed as concertinas linked at their respective ends by common end walls. In yet a further embodiment, the elements include substantially parallel rigid walls linked at their respective ends by flexible walls.

The retaining means may comprise a rim defined by elements forming the periphery of the cavity former such that the cover means locates within the rim and frictionally engages the peripheral elements. In alternative embodiments the retaining means is provided by a pair of side walls attached to the cover panel and which side walls frictionally engage the end walls.

In another embodiment of the first aspect of the invention, the support member may be nestable with at least one other similar support member to provide said stackability.

The member may comprise a top side surface complementary to an underside surface of the member. Open channels are preferably included in the member for receiving at least part of another member to enable stacking of the members.

In the preferred embodiment, a lattice arrangement is provided by an integrally formed matrix of cell walls, each cell wall having a V-shape in cross section, and wherein the matrix defines a plurality of cells. The structural strength of the support member may be improved by forming each cell with a floor on the same side of the member as the channels

The channels may form a lattice arrangement to permit stacking of members having non-identical configurations and having cell walls based on the same shape as the other members in the stack.

The cover means is preferably a panel or the like. However, in alternative embodiments, the cover means is reconfigurable between a relatively more compact state to facilitate transport of the cover means and an expanded state for engagement with the array of elements. Preferably, the cover means, in one state, is a blank that is foldable to form a housing, for receiving an array of elements, having an integrally formed lid engageable with the array for distributing said weight onto the array elements.

The cavity former may include means for retaining the cover means in engagement with the member. In the preferred embodiment, the retaining means is provided by walls depending from said lid, which walls frictionally engage the base to retain the lid in engagement with the member.

In a second aspect, the invention provides a cavity former comprising a plurality of linked modules, each module comprising:

-   -   (a) elements for supporting a predetermined weight of a person         standing on the elements or of wet concrete bearing down on the         elements until the wet concrete has cured sufficiently to         support itself; and     -   (b) cover means integrally formed with said elements for         distributing said weight onto the elements and preventing the         flow of wet concrete between the elements;     -   wherein each module is reconfigurable between a relatively more         compact state for stacking with other similarly configured         modules to facilitate transport of the module and an expanded         state for linking to at least one other module, the linked         modules in their expanded state defining a volume about which         the wet concrete may be poured thereby creating a cavity in the         resultant concrete slab.

Advantageously, cavity formers according to both the first and second aspects of the invention are made of a degradable material to facilitate disintegration of the cavity former over time to leave a cavity within the formed concrete slab. For example, the degradable material may be cardboard or fibreboard. If necessary, the cardboard or fibreboard may have a coating to slow the diffusion of moisture into the member and thereby prolong the weight bearing capacity of the member. In the preferred embodiment, the support member comprises paper pulp without a coating and the cover means comprises cardboard or fibreboard having a coating for slowing moisture diffusion.

In the expanded state the cavity former preferably has sufficient structural integrity to withstand up to 50 kPa from said weight of a person or wet concrete. The cavity former is preferably capable of supporting said weight of wet concrete for a period of about three hours after pouring despite diffusion of moisture into the elements from the wet concrete.

The cavity former is typically formed as a rectangle, e.g. a square, in its expanded state with a side length in the range 0.5 to 1.5 m, e.g. 1.0 to 1.2 m, and a height in the range of 150 to 400 mm, but preferably selected from 175, 225, 300 and 375 mm.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 is an isometric view of a cavity former in accordance with the present invention laid out with reinforcing materials in preparation for wet concrete to be poured to form a concrete slab or foundation;

FIG. 2 is an isometric view of a cavity former, according to a first embodiment of the first aspect of the present invention, in an expanded state and without a cover panel;

FIGS. 3A and 3B depict a side elevational view of panels used to form the cavity former in FIG. 2;

FIG. 4 is a cross-sectional view of the cavity former in FIG. 2 along the line A-A;

FIG. 5 illustrates a top schematic view of the collapsed state of the cavity former in FIG. 2;

FIGS. 6A and 6B depict an isometric view of a cover panel and base, in the expanded state, of a cavity former in accordance with a second embodiment of the first aspect of the present invention;

FIG. 7 is a top plan view of the base in FIG. 6B in a partially collapsed state;

FIG. 8 is an isometric view of a base in an expanded state of a cavity former in accordance with a third embodiment of the first aspect of the present invention;

FIG. 9 illustrates the base in FIG. 8 in a collapsed state;

FIG. 10 is an isometric view of a cavity former in an expanded state, with the array of elements shown in dashed lines, according to a fourth embodiment of the first aspect of the invention;

FIG. 11 is a plan view of a blank which is foldable to form the cover of the cavity former in FIG. 10;

FIG. 12 is an isometric view of a cavity former according to a fifth embodiment of the first aspect of the invention with the lid of the cover means open;

FIG. 13 is an isometric view of a support member of the cavity former in FIG. 12;

FIG. 14 is an isometric view of the underside of the support member of FIG. 13 with a floor formed in a cell;

FIG. 15 is a side elevation, in the direction A in FIG. 13, showing two stacked support members, the second support member is shown in dashed lines;

FIG. 16 depicts an isometric view of a cavity former in an expanded state in accordance with an embodiment of the second aspect of the invention; and

FIG. 17 is a plan view of a blank that is foldable to form a module of the cavity former in FIG. 16.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows cavity formers 10 in accordance with the present invention arranged with reinforcing products, such as reinforcing bars 12 and reinforcement mesh 14 supported on bar chairs 16, in preparation for pouring of the concrete to form a slab or foundation. It will be appreciated that the wet concrete is poured around and over the cavity formers such that the cured concrete slab has hollow sections in the shape of the cavity formers.

Referring to the first embodiment of cavity former 10 illustrated in FIGS. 2 to 5, it can be seen that the base 18 of the cavity former 10 has a support member comprising an array of elements in the form of panels 22 and 23 for supporting the weight of wet concrete until it is sufficiently cured to support itself, and a rim 24 formed on the peripheral panels 20 which operates to locate and retain the cover means on the panels 22 and 23. The cover means is provided in the form of a lid 34 (FIG. 4). It will, therefore, be appreciated that the weight of wet concrete on the lid 34 will be distributed to the panels 22, 23 along their upper edges.

The panels 20, 22 and 23 are preferably formed from a degradable material, which disintegrates over time to leave a cavity within the slab. The panels 20, 22 and 23 may otherwise be formed from any material having the required strength to support the weight of a worker standing on the cavity former or of the wet concrete poured to form the slab.

While FIG. 2 depicts the expanded state of the base 18, FIG. 5 illustrates the base 18 in a more compact state relative to the expanded state and which permits more convenient transport of the cavity former.

As illustrated in FIG. 4, the lid 34 is a rectangular panel dimensioned to snugly and frictionally engage the rim 24 so as to substantially prevent the ingress of concrete or cement into the volume defined by the cavity former 10.

A panel 22 in the first array is depicted in FIG. 3A. Each panel 22 has a series of slots 30 extending from one edge of the panel 22 inwardly to an extent approximating half the width dimension of the panel 22. Furthermore, the slots 30 have a width equivalent to the thickness of the panel 22 and panel 23. Two opposed peripheral panels 20 have the same configurations as support panel 22, but are formed with a greater width dimension to provide it with additional height in situ to form the rim 24.

A panel 23 in the second array is depicted in FIG. 3B. In these panels 23, most of the slots 30 extend from one side of the panel 23 inwardly to an extent approximating half the width of the panel 23, however two of the slots 30 proximate the respective ends of the panel 23 are located in the edge of the panel 23 which is opposite the edge having the remaining slots 30. The remaining two opposed peripheral panels 20 have the same configurations as support panel 23, but are formed with a greater width dimension to provide it with additional height in situ to form the rim 24.

In an assembled state, the panels 22 and 23 receive the perimeter panels 20 in the slots 30 proximate the ends of the panels 22, 23. In this manner, the perimeter panels 20 are interlocked with the remaining panels 22, 23 to form the base 18. Thus, a substantially square array of cells 28 is defined by the interlocking panels 22, 23.

The lid 34 is fitted to the base 18 to form the cavity former 10 by, firstly configuring the base 18 into the expanded state (FIG. 2) and placing the lid on the panels 22, 23 within the rim 24, as shown in FIG. 4. The rim 24 has dimensions marginally greater than the perimeter of the lid such that when the lid is located on the panels 22, 23 the outer perimeter of the lid 34 engages snugly and frictionally with the rim 24 to prevent the ingress of wet concrete into the cells 28 when the wet cement is poured over the cavity former 10. Such snug fitting also enables the lid 34 to be detached if required.

For ease and convenience of transport, the base 18 is collapsible to a more compact or collapsed state, as depicted in FIG. 5. This more compact state has a volume less than the expanded state and is generally flatter and, therefore, is a more convenient form for transport.

The construction of the cavity former 10 permits its division along any vertical plane parallel to either array of panels 22 or 23 or even along a diagonal, and such division will not affect the capacity of the cells 28, remaining after the division, to prevent the influx of wet concrete into the cells 28 and thereby, define a volume which will form a cavity in the slab. For instance, if the base was sectioned along the line A-A in FIG. 2 and the lid 34 was appropriately reshaped, the cells 28 remaining in the base 18 still define a volume which can be closed by the corresponding portion of the lid 34 and, therefore, are suitable for forming voids or cavities within a poured concrete structure.

Reference is now made to a second embodiment illustrated in FIGS. 6A, 6B and 7, in which like parts are indicated by like numerals preceded by a 1. Cavity former 110 comprises a base 118 and a cover means in the form of a lid 134. The base 118 has end walls 40 and support elements 42 located between the end panels 40. The support panels 42 are formed from a number of sections 46 joined via fold lines 44 to enable the support panels to expand and contract in length in a concertina-type effect. As can be seen from FIG. 7, the contraction of the support panels 42 enables the base 118 to collapse to a reduced volume for conveniently transporting the cavity formers 110. The support panels 42 have a bias toward the collapsed state.

The lid 134 in FIG. 6A comprises a panel 56, in the shape of a square, having opposed depending side walls 54. The cavity former 110 is formed by moving the end walls 40 apart to configure the base 118 into an expanded state thereby allowing the lid 134 to be applied such that the depending side walls 54 slot between the end walls 40 and operate to oppose the bias force provided by the support panels 42 and thus retain the base 118 in an expanded state. Fitted in this manner, the panel 56 of the lid 134 will rest on the upper edges 50 of the support panels 42 such that any weight placed on the plate 56 will be distributed to the support panels 134. Furthermore, the end walls 40 have an upper edge 48 spaced above the upper edges 50 of the support panels 42 by an amount equivalent to the thickness of the panel 56. Therefore, when the lid 134 is fitted to the base 118, and with the biasing provided by the support panels 42, the lid 134 is retained in snug engagement with the base 118 such that the ingress of wet cement below the lid 32 is avoided. It will, therefore, be appreciated that the lid 134 is detachable from the base 118.

As with the first embodiment of the cavity former 10, the second embodiment of the cavity former 110 may be sectioned along a vertical plane and yet retain its capacity to form a closed space. However, in this instance, the cavity former 110 may only be sectioned along a vertical plane parallel to the longitudinal axis (indicated by the arrow X in FIG. 6B) of the support panels 42. If the cavity former 110 is sectioned along a vertical plane that traverses the longitudinal axis X, for instance, an axis in the direction of arrow Y, the cavity former will no longer define closed spaces. Instead the cavity former 110 will be open for the ingress of wet cement during pouring. Accordingly, a cavity will not be formed in the concrete slab or foundation.

A third embodiment of the present invention is illustrated in FIGS. 8 and 9. in which like parts are indicated with the numerals preceded by a 2. As shown in FIG. 8, the cavity former 210 comprises a base 218 to which may be fitted the lid 134 in FIG. 6A.

The base 218 comprises end walls 62 and intermediate support panels 64 all of which are generally parallel and connected at their respective ends by a pair of flexible webs 66. Furthermore, the upper edges 62 of the end walls 60 are spaced above the upper edges 68 of the support panels 64 by an amount equal to the thickness of the plate 56 of the lid 134.

The base 218 can be re-configured into a collapsed state as depicted in FIG. 9 by moving each adjacent panel closer to its neighbour such that there is substantially no space between the panels 60 and 64. In this configuration, the base 218 has a substantially reduced volume relative to the expanded state shown in FIG. 8 and thereby enables the base 218 to be more conveniently transported.

The cavity former 210 is assembled by, firstly, configuring the base 218 into the expanded state and, secondly, then wedging the side walls 54 of the lid 134 between flanges 70 formed on the opposed end walls 60. In this manner, the wedging action of the side walls 54 acts to retain the base 218 in an expanded state whilst the lid 134 is snugly held in engagement with the end walls 60 and with the plate 56 engaging the upper edges 68 of the support panels 64 such that any weight placed on the plate 56 is distributed to the support panels 64. Such wedging action also permits the removal of the lid 134 from the base 218 such that the lid 134 may be regarded as detachable.

As with the second embodiment, the base 218 can be sectioned along a vertical plane parallel to the longitudinal axis of the support panels 64 without affecting the integrity of the closed volume defined by the remaining part of the base 218 and lid 134. Accordingly, the base 218 provides the same advantage in terms of reshaping the cavity former on the building site to form a cavity of the desired shape.

A cavity former 310 according to a fourth embodiment of the invention is shown in FIG. 10. In this embodiment, the cover means comprises a lid 320 and base housing 360 attached together by a hinge such that the lid 320 can fold over an array of elements 390 housed in the base housing 360. For transport purposes, the lid 320 and base housing 360 are integrally formed in a flat blank (FIG. 11) to reduce their volume.

The blank comprises a lid panel 321 linked to a base panel 361 via an intermediate wall 340. Walls 322 extend along opposed sides of the lid panel 321 adjacent to the intermediate wall 340. A further wall 326 extends along an edge of the lid panel 321 opposite the intermediate wall 340. Wall 326 is provided with opposed tabs 328 having an arcuate shape and including a slot 332 to define a tongue 330.

The lid 320 is formed by folding the blank such that edges 336 and 334 of walls 322 and 326 meet. During this process, tongues 330 of tabs 328 are inserted through slots 324 formed in walls 322 thereby to secure the walls 322 and 326 in depending arrangement from lid panel 321.

The base housing 360 comprises a square base panel 361 having attached thereto opposed side walls 362 and end walls 364 extending along an edge of the base panel 361 opposite intermediate wall 340. The end wall 364 is formed as a mirror image, including tabs 328, of wall 326 of the lid 320. Similarly, the intermediate wall 340 includes opposed tabs 342 formed identically to tabs 328. Side walls 362 include slots 372 and 374 respectively formed near opposed ends of the walls 362 for receiving the tongues of tabs 342 and 366. Each wall 362 further includes a flange 380 including equally spaced cut-out portions 382. The cut out portions 382 are shaped and spaced to receive one set of parallel elements 22 of the array 390.

The base housing 360 is formed by folding walls 362, 364 and 340 into an upstanding arrangement whereon the tongues of tabs 342 and 366 are inserted through slots 374 and 372 in walls 362. In this manner, the base housing 360 defines a tray for receiving the array of elements 390 therein. Each flange 380 is folded inwardly of the base housing 360. A first array of parallel elements 22 may then be inserted into corresponding opposed cut out portions 382 formed in the flanges 380. A second parallel array of elements 23 may be inserted into the base housing 360 to engage the first parallel array of elements 22 in the same manner as shown in FIG. 2. The cavity former 310 is then closed by folding the lid 320 over the array 390 such that depending walls 322 and 326 fit about and frictionally engage with walls 362 and 364 of the base when the lid panel 321 engages the top edge of the array of elements 390.

A cavity former 410 according to a fourth embodiment of the first aspect of the invention is shown in FIG. 12. The cavity former 410 has a cover means identical to the cover means in the third embodiment. The cover means is in the form of a housing that comprises a lid 320 and base housing 360 attached together by a hinge such that the lid 320 can fold over a cellular structure 420, which forms the support member, housed in the base housing 360. The lid 320 and base housing 360 are integrally formed in a flat blank (FIG. 11) to reduce the volume of the housing for transport purposes.

The housing and cellular structure 420 are made of degradable materials to ensure disintegration of the cavity former after the concrete slab has been formed. Such disintegration advantageously leaves a cavity in the formed concrete slab to accommodate unstable soils and to facilitate placement of electrical and plumbing conduits.

The cellular structure 420 is formed as an integral moulding of a suitable degradable material, for instance paper pulp, such that cell walls 430 define a square array of cells 436. The cell walls 430 have top edges 432 which collectively define a lattice on which the lid 320 rests when the cellular structure 420 is enclosed in housing. In this manner the weight bearing down on the lid 320 is distributed over top edges 432 of cell walls 430 for support by the cell walls 430. It will be appreciated that alternative cell wall 430 arrangements may be utilised to form cells 436 of different shapes and sizes. However, one advantage of discrete cells 436 is the potential for customized resizing of the cavity former 410 on the construction site as desired. This is because the cavity former 410 can be sectioned along any vertical plane without affecting the integrity of the remaining cells 436 to define a closed volume about which wet concrete can be poured to form a cavity of the desired shape in the subsequently formed concrete slab.

The cell walls 430 have an inverted V-shape to provide the cellular structure 420 with a top side 440 and an underneath side 450 that are complementary. The underneath of each cell wall 430 includes a channel 434 (FIG. 14) such that the underneath side 450 has a network of linked channels 434 into which a top side 440′ of another cellular structure 420′ (shown in dashed lines) is receivable for stacking (FIG. 15). In such arrangement, the stacked cellular structures 420 and 420′ nest to collectively define a volume that is less than the sum total volume defined by the individual cellular structures 420 and 420′ when separated.

For improved strength in the cellular structure 420, each cell 436 may be formed with a floor 438 (FIG. 15) formed in the underneath side 450 of the cellular structure 420 to be surrounded by the channels 434.

It will be appreciated that the network of linked channels 434 permits stacking of cellular structures having different cell wall 430 configurations in each array, provided that the cell walls 430 are in the same relative alignment with and are based on the same cell wall shape as cellular structure 420.

The internal angle of the V-shape forming the cell walls 430 may be in the range of 15-40°, but preferably in the range of 25-30°. In the cellular structure 420, the cell walls 430 have an internal angle of 27° for optimal weight distribution throughout the cellular structure 420.

An embodiment of the second aspect of the invention is shown in FIG. 16 as cavity former 510 comprising a series of linked modules in the form of upturned trays 520. Each tray 520 is transported as a blank (FIG. 17) and then formed into a tray 520 in situ.

The blank comprises a centre panel 530 formed as an elongate rectangle and flanked by walls 540 which include slots 542 at respective opposed ends of each wall 540. End walls 550 extend from respective opposite ends of the central panel 530. Each end wall 550 has a pair of opposed laterally extending tabs 552 that are formed identically to the tabs 328 of the third embodiment, i.e. to include a slot 554 defining a tongue 556.

Each tray 520 is formed by folding the blank such that edges 558 and 544 of the respective end walls 550 and walls 540 meet. During this process, the tongue 556 of each tab 552 is then inserted through corresponding slot 542 in walls 540 to secure the walls 540 and 550 in an upstanding relationship relative to the central panel 530.

A suitable cavity former for placement within a concrete slab is then formed by linking a plurality of aligned trays 520 together. The trays 520 may be linked by any suitable means, such as adhesive tape or stapling. In operation, the weight bearing down on centre panel 530 is distributed to the walls 540 and end walls 550 which support the weight.

Advantageously, cavity formers 510 can be made with varying sizes depending on the number of trays 520 linked together. Accordingly, such cavity formers 510 may be customised to the requirements of the concrete slab.

The cavity formers 10, 110, 210, 310 and 510 have support panels distributed throughout their internal volume to provide sufficient strength to withstand up to 50 kPa from the weight of a person or of wet cement or concrete for up to 3 hours. Similarly, the cellular structure 420, enclosed within the cavity former 410, provides the same strength for the same period of time. The structure of the cavity formers 10, 110, 210, 310, 410 and 510 ensures that the structural integrity is retained over the 3 hour period despite being made of a degradable material and the diffusion of moisture from the wet concrete into the cover and support panels which, over time, weakens them. Eventually the cover and support panels will disintegrate to leave a cavity in the slab.

A suitable degradable material of which to form the cavity former 10, 110, 210, 310, 410, 510 may be, for instance, uniform corrugated cardboard having a coating which slows the diffusion of moisture into the cardboard. A suitable coating must, firstly, be sufficiently moisture resistant to slow moisture diffusion to an extent that enables the cavity former to support the weight for up to 3 hours, i.e. until the concrete has sufficiently cured to be self supporting. Secondly, the coating must be substantially inert with the highly alkaline wet concrete. Finally, the coating must be able to withstand abrasions so that the integrity of the coating is not compromised in the event that particles in the wet concrete are ground against the coating, e.g. during pouring of the wet concrete or when a worker stands on the cavity former.

Examples of suitable coatings include plastics, either flexible or rigid, or paints, e.g. rymild white which is sprayed on and then cured by exposure to ultraviolet light.

In order to optimise costs, the cellular structure 420 may be formed integrally of low grade materials, e.g. paper pulp, without a coating and the housing 100 may be formed of high grade paper materials, for instance cardboard or fibreboard, with a coating.

The cavity formers 10, 1 10, 210, 310, 410 and 510 are generally rectangular, e.g. a square, but may equally be formed in any other polygonal shape as required and which can form a more compact state. The preferred embodiments are square and have a side length in the range of 0.5 to 1.5 m, but preferably 1.0 to 1.2 m. They may be formed with a height in the range 150-400 mm, but specifically may be fabricated with 175, 225, 300 or 375 mm heights.

It will be understood that the invention disclosed and defined herein extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention. 

1. A cavity former for use in forming a cavity in a concrete slab, said cavity former comprising: (a) a support member for supporting a predetermined weight of a person standing on the member or of wet concrete bearing down on the member until the wet concrete has cured sufficiently to support itself; and (b) cover means detachably engageable with the member for distributing said weight onto the member and preventing flow of said concrete into the member; wherein said support member is stackable with at least one other similar support member for transport, the stacked support members collectively defining a volume less than the summed volume of the separate members.
 2. A cavity former according to claim 1, wherein the support member comprises an array of elements that is reconfigurable between a relatively more compact state for stacking with other similarly configured arrays to facilitate transport of the array and an expanded state for engagement with said cover means, the expanded state defining a volume about which the wet concrete may be poured, thereby creating a cavity in the resultant concrete slab.
 3. A cavity former according to claim 2, wherein said elements pivotally intersect with each other such that the array is foldable between the more compact and expanded states.
 4. A cavity former according to claim 3, wherein each element of said array is a panel including slots formed to interlock with corresponding slots of other elements in the array to provide elements which pivotally intersect with each other.
 5. A cavity former according to claim 2, wherein said elements, in the expanded state of the array, define a plurality of cells which are divisible from the cavity former to thereby alter the size and shape of the cavity former without affecting the integrity of the volume defined by the remaining cells.
 6. A cavity former according to claim 2, wherein said elements of the array are panels that are elongate horizontally, in situ, arranged with their planes parallel or orthogonal in the expanded state.
 7. A cavity former according to claim 2, wherein the cavity former further includes means for locating the cover means in engagement with the array.
 8. A cavity former according to claim 7, wherein the locating means comprises a rim defined by elements forming the periphery of the cavity former such that the cover means locates within the rim and engages the peripheral elements.
 9. A cavity former according to claim 1, wherein the support member is nestable with at least one other similar support member to provide said stackability.
 10. A cavity former according to claim 9, wherein the support member comprise a top surface complementary to an underside surface of the member to provide said nestability.
 11. A cavity former according to claim 10, wherein the support member includes open channels for receiving at lest part of another support member to enable stacking of the members.
 12. A cavity former according to any one of claim 11, wherein a lattice arrangement is provided by an integrally formed matrix of cell walls, each cell wall having a V-shape in cross-section, said matrix defining a plurality of cells.
 13. A cavity former according to claim 12, wherein said matrix defining a plurality of cells is divisible to alter the size and shape of the support member without affecting the integrity of the volume defined by the remaining cells.
 14. A cavity former according to claim 13, wherein each cell has a floor formed on the same side of the support member as the channels to provide the support member with enhanced structural strength.
 15. A cavity former according to claim 14, wherein the support member comprises paper pulp.
 16. A cavity former according to claim 1, wherein the cover means is a panel.
 17. A cavity former according to claim 1, wherein said cover means is reconfigurable between a relatively more compact state, to facilitate transport of the cover means, and an expanded state for engagement with the support member.
 18. A cavity former according to claim 17, wherein the cover means, is in one state, a blank that is foldable to form a housing, for receiving said support member having an integrally formed lid engageable with the support member.
 19. A cavity former according to claim 18, wherein the cavity former further includes means for locating the cover in engagement with the support member.
 20. A cavity former according to claim 19, wherein the locating means comprises a hinge linking the lid to the housing.
 21. A cavity former according to claim 20, wherein means is provided for retaining the cover means in engagement with the support member.
 22. A cavity former according to claim 21, wherein the retaining means comprises walls depending from the lid, which walls frictionally engage with the housing when the lid is engaged with the support member.
 23. A cavity former according to claim 2, wherein the elements are formed as concertinas linked at their respective ends by common end walls.
 24. A cavity former according to claim 23, wherein the cavity former further includes means for locating the cover means in engagement with the array.
 25. A cavity former according to claim 24, wherein the locating means is provided by rims defined by the end walls having a height greater than the length of the elements.
 26. A cavity former according to claim 23, wherein means is provided for retaining the cover means in engagement with the array.
 27. A cavity former according to claim 26, wherein the retaining means is provided by a pair of depending side walls attached to the cover means, which side walls frictionally engage the respective end walls to retain the cover means in engagement with the array of elements.
 28. A cavity former according to claim 23, wherein the elements are formed to bias the cavity former into the more compact state, thereby enhancing frictional engagement of the side walls with the end walls.
 29. A cavity former according to claim 2, wherein the elements include substantially parallel rigid walls linked at their respective ends by flexible walls.
 30. A cavity former according to claim 29, wherein the cavity former further includes means for locating the cover means in engagement with the array.
 31. A cavity former according to claim 30, wherein the locating means comprises rims defined by common end walls linked to the flexible walls, which common end walls have a height greater than the elements.
 32. A cavity former according to claim 31, wherein means is provided for retaining the cover means in engagement with the array.
 33. A cavity former according to claim 32, wherein the retaining means is provided by a pair of depending side walls attached to the cover means, which side walls frictionally engage the respective end walls to retain the cover means in engagement with the array of elements.
 34. A cavity former according to claim 1, wherein the cavity former is capable of supporting up to 50 kPa from the weight of a person or of the wet concrete poured to form the slab.
 35. cavity former according to claim 1, wherein the cavity former is capable of supporting said predetermined weight for up to 3 hours.
 36. A cavity former for use in forming a cavity in a concrete slab, said cavity former comprising a plurality of linked modules, each module comprising: (a) elements for supporting a predetermined weight of a person standing on the elements or of wet concrete bearing down on the elements until the wet concrete has cured sufficiently to support itself; (b) cover means integrally formed with the elements for distributing said weight onto the elements and preventing the flow of wet concrete between the elements; and wherein each module is reconfigurable between a relatively more compact state for stacking with other similarly configured modules to facilitate transport of the module and an expanded state for linking to at least one other module, the linked modules in the expanded state defining a volume about which the wet concrete may be poured thereby creating a cavity in the resultant concrete slab.
 37. A cavity former according to claim 36, wherein each module, in the compact state, comprises a blank foldable to form an upturned tray.
 38. A cavity former according to claim 36, wherein each module includes means for retaining the module in the expanded state.
 39. cavity former according to claim 38, wherein the retaining means comprises tabs formed on the elements and slots for receiving the tabs.
 40. A cavity former according to claim 36, wherein the cavity former is made of a degradable material.
 41. A cavity former according to claim 40, wherein the cavity former is made of cardboard or fibreboard.
 42. A cavity former according to claim 41, wherein the cardboard or fibreboard has a coating which slows the diffusion of moisture into the cardboard or fibreboard.
 43. A cavity former according to claim 36, wherein the cavity former is capable of supporting up to 50 kPa from the weight of a person or of the wet concrete poured to form the slab.
 44. cavity former according to claim 36, wherein the cavity former is capable of supporting said predetermined weight for up to 3 hours. 